+1 overall; I will nitpick about example of "My English teacher..." as this is definitively a full disclosure of the source and is really not comparable to any political incorrectness or unfounded prejudice or personal aversions. It is a weak reference, but it is a reference. The phrase probably has bad rep because it is used as an argument only when someone runs out of better arguments (which tends to happen more when one is actually in wrong).
– UnreasonJul 14 '11 at 13:14

The problem is that all the dictionaries mentioned change every couple of years as more evidence comes in from corpus studies. And the beauty of English is that there is no single authority. In fact many academics now speak of "Englishes" rather than "English", because there are so many varieties, and they are all correct in their own context.

nohat has listed lots of appropriate references. I'm a fan of Language Log and OED myself. I also think Google is a great tool, at least for comparison purposes.

I don't think a reference to one of these is necessarily important, though it lends credibility. More than anything, I think an appeal to one of these sources should provide the ability to find more information on the topic.

I also think we will find at least some situations haven't yet been addressed by these credible authorities. In these cases, do you reference academic papers if possible? How far should you attempt to answer a question on your own—can tweaking and reviewing a home-grown answer be a community responsibility? I would say yes, especially so in the case of grammaticality judgements, where native speaker intuition is infallible.

Subjectivity in areas of debate will be unavoidable, and permissible to some extent. E.g., "How do I pronounce x" would be fine, but "is this better than x" is obviously something to be closed. I think StackOverflow's rules would work fine: "Avoid asking questions that are subjective, argumentative, or require extended discussion."

I believe any "answer" about the English language IS subjective, and can often require extended discussion. English isn't an atomic language. It is very large, very varied, and very fast changing.
– Vincent McNabbAug 10 '10 at 2:06